The thin air, yes. Many guide books and travel blogs warn you about doing what we do, travel to Cusco by air. At more that 3 300 meters above sea level the city has an air pressure and oxygen level alien to our lowland bodies. Almost immediately we experience a light dizziness and we start panting from the slightest effort. Like just moving our feet. We see our fellow passengers proceed slowly and sort of dignified between the arrival entry and the luggage conveyor belts. It seems wise, so we mimic their movements.
In the coming days we plan to take things really easy and rest a lot, to adapt to the height. We figure we need to be fit when we go up to Machu Picchu three days later. But resting here is not so easy. The body chemistry is in imbalance which makes it really hard to sleep. I find myself regularly gasping for air, leaving me upset and awake.